1. Field of the Invention
The invention relates to a process for preparing salts of polyaspartic acid by treating polycondensates of aspartic acid in aqueous medium with bases, employing at least 0.7 equivalent of an alkali metal and/or alkaline earth metal base per mole of the aspartimide, aspartic acid and/or aspartamide units present in the polycondensates, and to their use in detergents and cleaners.
2. Discussion of the Background
Thermal polycondensation of aspartic acid at temperatures above 190.degree. C. results in polycondensates which essentially comprise aspartimide units. Journal of Medicinal Chemistry, 16, (1973) 893-897 discloses the condensation of aspartic acid in the presence of phosphoric acid as catalyst. WO-A-93/24661 discloses that acids such as phosphoric acid, phosphorous acid, hypophosphorus acid, hydrochloric acid or sulfuric acid, and sodium bisulfate or potassium bisulfate, catalyze the polycondensation of aspartic acid at temperatures above about 150.degree. C. The polycondensates obtainable in this way predominantly comprise aspartimide units and may, depending on the temperature at which the polycondensation was carried out, comprise aspartic acid, aspartamide and aspartimide units. Treatment of polycondensates of aspartic acid with alkali metal bases, e.g. sodium hydroxide solution, results in the corresponding salts of polyaspartic acids.
Polyaspartimides can also be prepared by addition of ammonia onto maleic acid and condensation. Hydrolysis and neutralization of the polyaspartimides with sodium hydroxide solution results in the sodium salts of polyaspartic acids. Up to 1 mol of sodium hydroxide solution is employed per mole of aspartimide unit in the polycondensates. Processes of this type are disclosed, for example, in WO-A-93/23452, WO-A-94/3526, WO-A-94/3527 and DE-A-36 62 672. The preparation of alkali metal salts of polyaspartic acids is disclosed in WO-A-92/16462, WO-A-92/16463 and U.S. Pat. No. 5,152,902, wherein the polyaspartimide obtainable by condensing aspartic acid is suspended in water and, while monitoring the pH, sodium hydroxide solution is metered in so that the pH does not exceed 9.5. pH measurement in industrial apparatus is inadvisable where the viscosities are high and the solids contents of the mixtures to be hydrolyzed are high, because pH electrodes usually do not display accurate values because of the delay in mixing.
EP-A-0 581 452 discloses the preparation of sodium salts of polyaspartic acid by adding sodium hydroxide solution dropwise to an aqueous suspension of polyaspartimide which has been heated to 50.degree.-60.degree. C. in such a way that the pH of the reaction mixture does not exceed 10. After the addition of sodium hydroxide solution, the reaction mixture is kept at 50.degree.-60.degree. C. for 1 hour more. About 83% of the amount of sodium hydroxide solution necessary for complete neutralization is employed.
Alkaline hydrolysis and neutralization of polysuccinimides and polyaspartic acids is a highly exothermic process which must be reliably controlled when the process is carried out on the industrial scale in order to ensure straightforward operation and uniform product quality.